Most of the raw feed used for the production of strategic titanium metal pigment is imported. Development of a process to extract titanium and associated values from domestic ore resources would decrease foreign dependency as a source of strategic titanium metal which is used in many industries in the United States.
A review of the literature shows that very little research has been reported on the processing of calcium-containing ores, such as perovskite, in order to recover the titanium. Perovskite ore generally refers to an ore containing calcium and titanium as calcium titanate in sufficient quality and quantity so as to be capable of being a source of calcium and titanium or derivatives thereof. A perovskite concentrate is a perovskite ore that has been subjected to a beneficiation process for removal of waste material, such as gangue and slimes.
There is substantial prior art on the recovery of pigment-grade titanium dioxide from other ores, such as ilmenite or rutile. Thus, U.S. Pat. No. 2,287,861 discloses the production of titanium oxide pigments by digesting ilemenite ore with sulfuric acid to form a solid mass containing titanium sulfate, iron sulfates, and unreacted ore, and gangue. This digest cake is then water treated to form a mass comprising titanium sulfate and ferric sulfates in solution and undissolved solids in suspension. The liquor is then clarified to separate unreacted ore, gangue, and slime residues; and the clarified liquor, which is stock titanium sulfate solution, is subjected to hydrolysis to produce solid crude metatitanic acid. The metatitanic acid may then be calcined at high temperatures to form the final titanium oxide pigment product.
Various improvements can be found in the prior art for conducting this process. Thus, in U.S. Pat. No. 3,397,037, a surface active agent is employed in the leaching step to decrease the time necessary to effect satisfactory leaching. In U.S. Pat. No. 4,321,236, the leaching step is said to be improved by heating the titanium material and mineral acid reactants to an elevated temperature prior to mixing and then mixing in a leaching zone and further heating to a higher temperature. This process is said to cause at least a portion of acid soluble titanium values in the titanium material to be hydrolyzed and removed by precipitation.
Some prior work has been conducted on the recovery of titanium from perovskite ores. Thus in Russian work reported by Shtern, J. Applied Chem., 11, pp. 1155-1160, perovskite containing, in percent: 40.8 TiO.sub.2, 30.7 CaO, and 5.2 Fe.sub.2 O.sub.3, was sulfated by acid baking with twice its weight of 93 pct H.sub.2 SO.sub.4 at 170.degree.-150.degree. C. for 1 h and then at 150.degree.-125.degree. C. for 1 h, which resulted in 90-93 pct TiO.sub.2 sulfation. Applying this method, however, in the treatment of domestic perovskite concentrate resulted in less than 80 pct TiO.sub.2 sulfation. The domestic perovskite having a lower CaO:TiO.sub.2 ratio is more refractory and not as readily sulfated by acid baking. In work conducted and reported in Russian journals by Goroshchenko et al, Sbornik Trudov Khim., Tekhnol. Mineral. Syrlya, Kolsk. Poluostrova, 1959, No. 1, pp. 25-39, a sulfuric acid-ammonium sulfate fusion of the perovskite ore is employed followed by leaching and precipitation. This process, however, is reagent intensive, the fusion mass is thick and sticky, and the reaction time is several hours.
In a process conducted by the U.S. Bureau of Mines and reported by Elger et al, Report of Investigations 8497, Bureau of Mines, 1980, page 20, perovskite ore is reacted with carbon at high temperatures in an electric furnace to produce titanium carbide which is then chlorinated to titanium tetrachloride. This method is energy intensive, and the carbide form has an extremely high melting point which creates physical handling problems.
In a further method published by Goroshchenko et al, Invest. Karelski Kolsk. Filial, Akad. Narik S.S.S.R., 1959, No. 4, pp. 135-141, a sulfuric acid digestion is employed wherein the digestion reactor contains some ammonium sulfate. This is followed by an extraction step. In this process, the titanium is eventually precipitated as a double salt of ammonia. This process, however, is complicated and reagent intensive since all excess acid is converted to ammonium sulfate by ammonia. Neither of the above-identified Russian studies by Goroshchenko et al indicate conditions wherein sulfuric acid alone could be used to solubilize the titanium.
A further approach known in the art for treating perovskite involves roasting at 1200.degree. C. in hydrogen sulfide gas, followed by leaching to remove calcium and iron sulfides and leaving the titanium dioxide. The disadvantages of this process are high temperatures and use of a highly toxic gas. This work is described, for example, by Kelly et al in Canadian Pat. No. 1,052,581, issued Apr. 17, 1979.
A further procedure known in the art for treatment of perovskite ore is by leaching with nitric acid in an autoclave. This is also an expensive and corrosive process. This work is published by the Russian authors Kestriken et al, Syre, Vses. Nanch-Issled, Inst., Miner. Syrga, No. 13, 1966, pp. 63-69.
It is, therefore, apparent that a need remains in the art for a method which is effective to extract titanium from its combination with calcium in ores, such as perovskite ore, using simplified and efficient processes.